1. Field of the Invention
The present invention relates to a sealing gasket for closure, made of a polyurethane elastomer having a low hardness and causing no yellowing, as well as to a process for producing a closure using such a sealing gasket.
2. Related Prior Art
Many sealing gaskets for metal closure are made of a material using a polyvinyl chloride resin as a main component, for the good workability and moldability.
In these metal closure sealing gaskets, the polyvinyl chloride resin contains a plasticizer (typified by dioctyl phthalate) in a large amount and, therefore, there has been a problem that the plasticizer dissolves in the oil and fat, etc. with which the sealing gasket comes in contact. Against this dioctyl phthalate, there is a doubt that it may be an endocrine-disrupting substance. As a countermeasure therefor, a metal closure sealing gasket made of a polyvinyl chloride resin composition containing a derivative of an acyl oxycarboxylate as a plasticizer is described in, for example, JP-A-58-67780.
Also, in recent years, generation of dioxin in combustion of polyvinyl chloride resin for its disposal has become a social problem.
Meanwhile, molded polyurethane elastomers have excellent physical properties, for example, high tensile strength, fatigue resistance, good low-temperature flexibility, and abrasion resistance. Because of these properties, polyurethane elastomers are in use in production of rolls, packings, various machine parts, automobile parts, electronic apparatus parts, etc. Further, polyurethane elastomers are a high polymer material having very high biocompatibility to human body and, therefore, are also in use as a catheter, an artificial blood vessel, an artificial heart, an artificial kidney, etc.
As the process for producing a polyurethane resin, there is generally known, for example, a process of reacting a high-molecular polyhydroxyl compound, a low-molecular polyhydroxyl compound (a chain extender) and an organic polyisocyanate in the presence or absence of a catalyst.
This process is largely classified into two processes. One is a one-shot process of subjecting the above-three components to simultaneous reaction and casting; and the other is a prepolymer process of beforehand reacting a high-molecular polyhydroxyl compound and an organic polyisocyanate to form an isocyanate-terminated urethane prepolymer and then subjecting this prepolymer (as a main material) to a crosslinking reaction with a low-molecular polyhydroxyl compound.
As this prepolymer process for producing a polyurethane resin, there is disclosed, in, for example, JP-A-63-8685, a process which comprises reacting a polyester polyol with diphenylmethane diisocyanate to form an isocyanate-terminated urethane polymer and then subjecting this prepolymer (as a main material) to a heat treatment of 140° C. with a mixture (as a crosslinking agent) of 1,4-butanediol and trimethylolpropane to obtain a polyurethane elastomer having a hardness (JIS A) of 78 to 80. The polyurethane elastomer obtained by this process has excellent mechanical strengths and is well balanced as a cleaning part for electronic photocopier, in particular.
As the technique of using a polyurethane resin as a sealing gasket for a closure of a container (e.g. a pail or an open drum), there is, for example, a technique described in JP-A-61-9481. In this technique, a first solution containing a urethane prepolymer as a main component and a second solution containing a polyol as a main component are mixed to obtain a mixture having a controlled viscosity of 200 to 20,000 mPa·s at 0 to 60° C.
In the above technique disclosed in JP-A-61-9481, as the polyisocyanate compound used for obtaining the urethane prepolymer, there are mentioned tolylene diisocyanate, diphenylmethane diisocyanate, etc. In the Examples of the literature, there is used an isocyanate-terminated prepolymer obtained by reacting tolylene diisocyanate and a bifunctional polypropylene glycol.
Polyurethane elastomers using such an aromatic isocyanate have a long history. In the casting by the prepolymer process, a short-chain glycol such as 1,4-butanediol, trimethylolpropane or the like is used as a curing agent for diphenylmethane diisocyanate-based urethane prepolymer, and an amine compound typified by CUAMINE or MOCA is used as a curing agent for tolylene diisocyanate-based urethane prepolymer. A polyurethane elastomer produced by curing with MOCA has a long history, but MOCA is feared for its possible carcinogenicity.
Polyurethane elastomers using an aromatic isocyanate have excellent mechanical strengths. However, when they are produced so as to have a low hardness, specifically a JIS A hardness of 75 or less, they are extremely inferior in mechanical properties and have had a problem particularly in tensile strength. Further, since aromatic polyisocyanates cause yellowing with the lapse of time, it was impossible to produce a molded polyurethane elastomer causing no yellowing, using an aromatic isocyanate.
Furthermore, when a polyurethane elastomer is used in hygienic storage of food, the aromatic isocyanate used as a component of the polyurethane elastomer is hydrolyzed to become an amine compound. For example, 4,4′-diphenylmethane diisocyanate, when hydrolyzed, becomes 4,4′-diaminodiphenylmethane. This compound is ordinarily called DAM and has a very high carcinogenicity. Therefore, it is not preferred to use an aromatic amine or a compound which may become an amine terminal when hydrolyzed, for production of a polyurethane elastomer used in hygienic storage of food.